1. Field of the Invention
This invention relates in general to data storage systems, and more particularly to a method, apparatus and program storage device for providing asynchronous status messaging in a data storage system.
2. Description of Related Art
Business organizations often insist on continuous availability to critical data twenty four hours a day, every day of the year. For many businesses and organizations, such as banks and other financial institutions, the inability to access data for any period of time could lead to loss of financial transactions, financial losses, loss of credibility and reliability with customers, and maybe even a total failure of the business. Therefore, the ability to provide continuous availability to data for the mission critical applications is often a necessity for business survival.
Generally, data backup systems copy a designated group of source data, such as a file, volume, storage device, partition, etc. If the source data is lost, applications can use the backup copy instead of the original, source data. The similarity between the backup copy and the source data may vary, depending upon how often the backup copy is updated to match the source data. If the backup copy is updated in step with the source data, the copy is said to be a “mirror” of the source data, and is always “consistent” with the source data.
As can be seen, a key feature of any disaster recovery system is the backup of critical data and updates to such backed-up data as soon as possible. The simplest method of backing up data is with off-site backups. However, the extent of successful data recovery in the event of failure is dependent upon the frequency of updates to the backed-up data. One data back-up system is the Peer-to-Peer Remote Copy (PPRC) function offered by International Business Machines, Corporation (IBM), the assignee of the subject patent application. PPRC provides synchronous copying of data from a primary system to a remote (secondary) system. With PPRC, primary storage devices are coupled to counterpart secondary devices by suitable communications links. Often, both the primary and secondary systems include a non-volatile storage unit (NVS), such as a battery backed up storage unit, to maintain data in the event of a power or system failure. With PPRC, when data is written to the primary NVS, the data is then transferred to the NVS in the secondary controller. At later points in time, the data in the primary and secondary NVSs will be destaged to the attached storage devices. With PPRC, there is minimal risk of losing data upon the occurrence of a failure because data is at the secondary controller synchronously with updates to the data at the primary DASD managed by the primary controller.
The primary and remote system may be connected by using a proprietary system, such as Enterprise System Connection (ESCON®) or Fibre Connection (FICON™), or by using an open system. An open system is a system whose characteristics comply with standards made available throughout the industry, such as SCSI and fibre-channel (FC) systems. Proprietary systems use an interrupt protocol to provide synchronous operation between the two systems thereby facilitating messaging and event notification between the two systems. However, open systems, such as SCSI and fibre-channel, do not use interrupts. A device that begins a SCSI transaction issues a command to another device (the SCSI target) thereby giving the SCSI target a task to perform. Typically a SCSI host adapter is the initiator, but targets may also become initiators. Still, the primary and secondary system connected by a physical connection or fibre channel operate asynchronously. Thus, asynchronous events on a SCSI controller cannot be returned to a host initiator or peer subsystem initiator. Accordingly, such events go undetected by the initiator until a command completes or terminates.
It can be seen then that there is a need for a method, apparatus and program storage device for providing asynchronous status messaging in a data storage system.